Converting Image to bitmap turns background black

Don't save as a bitmap file. The file format doesn't support transparency, so the image will be saved without transparency.

You can use the PNG file format instead. That will preserve the transparency.

If you really need it to use the bitmap file format, you have to make it non-transparent first. Create a new bitmap with the same size, use the Graphics.FromImage method to get a graphics object to draw on the image, use the Clear method to fill it with the background color that you want, use the DrawImage method to draw your image on top of the background, and then save that bitmap.